1. Field of the Invention
This invention relates to hydrogen storage materials and their electrochemical application. More particularly, this invention relates to the composition of novel materials for rechargeable hydride electrode materials. This invention further relates to a simple but effective method to determine a multi-component alloy as a potential candidate for hydride electrode applications.
2. Description of the Prior Art
Hydrogen can be stored in a heavy cylinder at high pressure as a gas at room temperature, or it can be stored in a well insulated container at low pressure as a liquid at ultra low temperature. The high pressure storage method involves significant safety problems, and relativey little hydrogen can be stored in a given volume of container vessel. The ultra low temperature storage method involves a significant waste of electricity to power cryogenic liquefaction devices, and, because of evaporation, the hydrogen cannot be stored indefinitely.
A preferable way to store hydrogen is to use a solid material which can absorb hydrogen in a reversible manner. This process is known as hydriding. Two examples of hydriding processes are: EQU M(s)+1/2H.sub.2 (g).fwdarw.MH(s) (1) EQU M(s)+1/2H.sub.2 O+e.sup.-- .fwdarw.MH(s)+OH.sup.-- ( 2)
where M(s) is the solid hydrogen storage material, MH(s) is the solid hydride, e.sup.-- is an electron and OH.sup.-- is the hydroxyl ion. Equation (1) is a solid-gas reaction process which can be used to store thermal energy. Equation (2), on the other hand, is an electrochemical reaction that can be used to store electrical energy. In both equations, hydrogen is stored during a charge reaction and is released during a discharge reaction.
Not every metal alloy can be used in the above hydriding process. It is also the case that not every metal alloy that can be utilized in the solid-gas reaction (Eq. 1) can be used in the electrochemical reaction (Eq. 2). For example, the hydrogen storage materials: Ti-Zr-Mn-C-V alloys, disclosed in U.S. Pat. No. 4,160,014 are not readily suitable for electrochemical reactions, as for example those involved in a battery application. Another example of hydrogen storage materials is given in Japanese Patent Sho 55-91950 which discloses alloys with the following composition formula: (V.sub.1-x Ti.sub.x).sub.3 Ni.sub.1-y M.sub.y, where M equals Cr, Mn, Fe, and where x and y are defined by: 0.05.ltoreq.x.ltoreq.0.8 and 0.ltoreq.y.ltoreq.0.2. These materials restrict the amount of Ni+M equal to 25 atomic percent with less than 5 atomic percent of M, and the amount ot Ti+V equal to 75 atomic percent. As a result, in addition to the potential corrosion problem adduced from using these materials, the hydrides of these materials are either very stable at ambient temperature or are of high cost. Consequently, these materials are not readily useable for electrochemical applications.
Among the many hydride materials that have been developed, only a few of them have been tested electrochemically. Examples of such research are U.S. Pat. Nos. 3,824,131, 4,112,199, and 4,551,400. The hydride electrode materials invented primarily by the present inventor and disclosed in U.S. Pat. No. 4,551,400 have superior properties as compared to the hydride electrode materials described in the other patents hereinabove cited. The materials disclosed in the U.S. Pat. No. 4,551,400 are grouped as: ##EQU1## These materials are all limited to the pseudo TiV.sub.2 type alloys with the following composition restriction:
Group (a): Ti=33.3 atomic %, V+Ni=66.7 atomic %; PA1 Group (b): Ti+Zr=33.3 atomic %, V+Ni=66.7 atomic %; and PA1 Group (c): Ti+Cr=33.3 atomic %, V+Ni=66.7 atomic %. PA1 Excellent hydrogen storage capacity; PA1 superior electrochemical catalyst for hydrogen oxidation; PA1 high hydrogen diffusion rate; PA1 suitable hydrogen equilibrium pressure; and PA1 reasonable cost.
This restriction results in all these materials having one or several weaknesses, especially high cost, short life cycle, and low capacity, as well as in some cases poor rate capability.
A good hydrogen storage material of the class described suitable for electrochemical applications has not been reported to date in the scientific literature, as well as Letters Patent. Particularly there has been no disclosure of how to provide a simple qualitative approach for developing or optimizing hydride materials for storing hydrogen as well as for hydride electrodes. As a result, the common method has been one of a trial-and-error, which has resulted in the expenditure of considerable wasted time, money and human resources.
Consequently, which is needed is a good hydrogen storage electrode material, having at the minimum the following properties:
To fit the above restrictions, the present invention provides, through the application of thermodynamics, kinetics and electrochemistry, a method for selecting a good hydride candidate suitable for electrochemical applications. More particularly, the composition of advanced hydride electrode materials and the methods of their fabrication are disclosed herein.